1. Field of the Invention
The present invention relates generally to a burst-synchronous CDMA (Code Division Multiple Access) communication system, and more particularly to a joint detection receiver and its control method, irrespective of a length of an orthogonal code.
2. Description of the Related Art
Prior to describing the present invention, it should be noted that the present invention is applicable to all kinds of communication systems for adapting orthogonal codes to discriminate between physical channels for use in a communication system.
An orthogonal code indicates a prescribed code having a predetermined length “n” (where, n≧1), and a total number of orthogonal codes each having the predetermined length “n” is set to “n”. According to a characteristic of such an orthogonal code, if one specific orthogonal code is time-synchronized with other orthogonal codes, a cross correlation value between the specific orthogonal code and N−1 number of other orthogonal codes other than the specific orthogonal code becomes zero. Therefore, the orthogonal code has been widely used to discriminate between physical channels in a wired or wireless communication system.
For ease of description, a system matrix configuration method and its associated application for use in an inventive joint detection receiver will hereinafter be described using an exemplary Narrow Band Time Division Duplex communication scheme.
A representative example of a third generation asynchronous mobile communication system is a WCDMA (Wideband Code Division Multiple Access) system for providing a user with a voice service and a packet service. The WCDMA system is classified into a FDD (Frequency Division Duplex) system for separating a transmission frequency and a reception frequency from each other, and a TDD (Time Division Duplexing) system for adapting the same frequency as transmission/reception frequencies. The TDD system is classified into a WB-TDD (WideBand TDD) system using a chip rate of 3.84 Mcps (Mega chip per second) and a NB-TDD system using a chip rate of 1.28 Mcps. The WB-TDD and NB-TDD systems each use a specific time period called a timeslot to discriminate between uplink and downlink transmissions, however, they use an orthogonal code in the timeslot to discriminate between different channels. The orthogonal code for use in the NB-TDD and WB-TDD systems is called an OVSF (Orthogonal Variable Spreading Factor), and adjusts a length of the orthogonal code according to an amount of transmission data, thereby enabling the data transmission.
The present invention relates to a joint detection receiver for a mobile communication system, and more particularly to a method for controlling a joint detection receiver adapting an OVSF of one timeslot to a TD-CDMA system.
FIG. 1 is a schematic diagram illustrating a radio frame (also called a frame) structure having a predetermined length of 10 ms currently adapted as a basic unit of physical channel transmission in the NB-TDD system, a basic structure of a DPCH (Downlink Physical CHannel), a DwPCH (Downlink Pilot CHannel) structure, and their respective positions. The frame 101 has a predetermined length of 10 ms and 12800 chips, and is composed of two sub-frames 102. Each sub-frame 102 is one of the two sub-frames contained in the frame 101, and these two sub-frames have the same internal configuration. The sub-frame 102 has a predetermined length of 5 ms, and includes seven timeslots 104, a DwPTS (Downlink Pilot Time Slot), an UpPTS (Uplink Pilot Time Slot) 106, and a GP (Guard Period) 105. Each time slot is composed of 864 chips, and is adapted for UL (UpLink) or DL (DownLink) transmission. An upward arrow indicates a UL time slot, and a downward arrow indicates a DL time slot.
The NB-TDD system determines how many time slots from among the 7 time slots contained in one sub-frame can be used for the DL or UL transmission. A first time slot (TS #0) 103 must always be assigned with a DL, and a second time slot (TS #1) must always be assigned with a UL. The DwPTS 104 having 96 chips, the GP 105 having 96 chips, and UpPTS 106 having 160 chips exist between the first time slot TS#0 and the second time slot TS#1. The DwPTS is adapted to search for an initial cell and perform synchronization and channel estimation. The UpPTS is adapted to perform channel estimation at a base station (BS), and to establish uplink synchronization with a UE (User Equipment). Two time slots of the GP are set to DL and UL time slots, and are adapted to remove signal interference caused by multi-path delay between two signals. A switching point is adapted to discriminate between the UL and DL time slots. The NB-TDD system contains two switching points in its one sub-frame. One of the two switching points is positioned between the DwPTS and the UpPTS, and the other one is variably positioned according to a time slot allocation status.
It is assumed that a downlink physical channel 107 is set to a physical channel positioned in the first time slot TS#0, and an uplink physical channel has the same configuration as the downlink physical channel 107. The downlink physical channel includes a data symbol area 109, a midamble area 110, a data symbol area 111, and a GP area 112. Each data area of the downlink physical channel is composed of 352 chips, and transmits data using an SF (Spreading Factor) 16. The uplink physical channel having the same configuration as the downlink physical channel may use a plurality of SFs 1, 2, 4, 8, and 16. The number of physical channels or user channels distinguishable by the OVSF may be set to “k” (where, k=1, 2, . . . , 16).
Each data area is multiplied by a scrambling code of a base station (BS) of the NB-TDD communication system, and performs data transmission. The scrambling code is multiplied by the data area in chip units, and individual base stations use the same scrambling code in UL and DL transmissions. There are two kinds of scrambling codes in the scrambling code for every base station. One of the two scrambling codes is a scrambling code for an even frame, and the other one is a scrambling code for an odd frame. If time synchronization of the OVSF is not performed at regular intervals, an auto-correlation characteristic of signals is deteriorated, such that the scrambling code is adapted to reduce such deterioration of the auto-correlation characteristic and discriminate between a signal of its-related base station and other signals of other base stations.
The midamble area 110 serves as a kind of training sequence. More specifically, a specific code is selected from among a plurality of codes according to its use by means of a computer or other methods, and the midamble area 110 is created using the specific code. Each base station of the NB-TDD communication system uses a unique midamble code. The unique midamble code is created by moving a predetermined midamble code at intervals of a predetermined time using a specific basic code.
In case of the DL time slot, the midamble code is adapted to estimate a radio channel impulse response between the base station and the UE. More specifically, the midamble code is adapted to estimate a channel environment between the base station and the UE, and is adapted to recognize information of channels transferred from the base station to the UE.
In the UL time slot, the base station analyzes the midamble code to recognize which one of UEs transmits a channel signal, and the midamble code is adapted to estimate a channel environment between the UE and the base station, i.e., an impulse response of a wireless environment. The GP area 112 positioned at the end of the time slot is composed of 16 chips, and is adapted to remove signal interference between different time slots.
The DwPTS illustrated in FIG. 1 includes a GP area 113, and a synchronous-downlink (SYNC-DL) code 114. The GP area 112 of a previous time slot TS#0 and the GP area 113 creates a GP having a length of 48 chips, and this created GP having 48 chips is adapted to remove signal interference caused by a multi-path delay between the TS#0 and the DwPTS. Because the SYNC-DL code 114 being a firstly-found signal of the UE finds an initial cell, and establishes synchronization with the found cell, the above-created GP having 48 chips is determined to have a relatively long period of time and plays a very important role in a communication system. If signal interference occurs between the TS#0 and the GP area, a GP having a short period of time may be seriously affected by the signal interference. Therefore, the sum of the GP 112 positioned at the end of the TS#0 and the GP 113 of the DwPTS creates a new GP having 48 chips, thereby guaranteeing an accurate reception of the SYNC-DL code.
The SYNC DL code 114 is a signal to be firstly found by the UE, and there are 32 kinds of SYNC DL codes. The UE performs correlation between 32 kinds of code words and the strongest signal to determine the SYNC DL code, and establishes synchronization with its-related cell.
A conventional detector for use in a current mobile communication system is called a single-user receiver, which utilizes a detection technique that detects only a desired signal of one single user in a communication system and regards all the other undesired user signals and interference signals as a noise signal, respectively. For example, a matched filter detector serving as a linear filter designed to maximize an output SNR (Signal-to-Noise Ratio) for a given input signal is a single-user receiver, is not efficiently resistant to an MAI (Multiple Access Interference) and an ISI (Inter Symbol Interference) because both the MAI and the ISI are regarded as noise signals, and does not use any knowledge associated with the aforementioned mobile channel or signature sequence.
Recently, a new technique for improving the single-user receiver has been developed. A representative example of the new technique is a joint detection receiver for efficiently removing the MAI and the ISI, and is schematically illustrated in FIG. 2. Although it is assumed that the joint detection receiver is positioned at the base station in FIG. 2, the joint detection receiver may be positioned at the UE.
Referring to FIG. 2, the joint detection receiver is based on a joint detection method, which efficiently removes the MAI and the ISI while increasing the capacity of a communication system, and simultaneously detects a plurality of users of the communication system to efficiently remove the MAI and the ISI. When receiving multiple user signals at the joint detection receiver, the joint detection receiver can estimate channel impulse responses of the received multiple user signals and their multi-path signals because the detection method can simultaneously detect multiple users. Transmission and reception of the joint detection receiver are schematically illustrated in FIG. 2 for the purpose of explanation. The joint detection receiver can include a channel estimation unit 200, a joint detection unit 201, a channelization code generator 203, and a scrambling code generator 204. Individual mobile stations (MSs) 205, 206, 207, and 208 can be assigned one or more OVSF channelization codes 211, 212, 213, 214, 215, and 216 for transmitting their own signals from antennas 227, 228, 229, and 230, respectively.
The joint detection receiver is located in a BS (Base Station) 231 for receiving a signal via its own antenna 232, where the signal is transmitted from the several MSs 205, 206, 207, and 208 via antennas 227, 228, 229, and 230. The channel estimation unit 200 generates radio channel impulse responses 223 and 224 for the joint detection unit 201, where the radio channel impulse responses 223 and 224 are estimated from the received midamble code signals. The joint detection unit 201 can also be located in the MSs 205, 206, 207, and 208, respectively. The joint detection unit 201 can be basically divided into two parts, i.e., a system matrix generation block 209 for joint detection and a solution block 210 for the system matrix, such that all the desired user signals 217, 218, 219, 220, 221, and 222 transmitted within the same time slot can be detected. In this case, the solution block 210 for the system matrix is generated from the radio channel impulse responses 223 and 224, the channelization code 225, and the scrambling code 226.
The joint detection receiver can be used for a TD-CDMA communication system characterized by burst-synchronous transmission/reception in the same time slot. Because the joint detection receiver can exploit prior knowledge about both the channelization code the radio channel estimation in order to mitigate the MAI and ISI from the same time slot, it can simultaneously regenerate all the desired signals within the same time slot.
The joint detection receiver for estimating all the received signals and their multi-path signals is characterized in that it has high complexity, that is, higher than that of a single-user receiver. The complexity of the joint detection receiver is evaluated in term of the number of multiplications and the number of additions for solving system matrix equations describing the joint detection method. The complexity of the joint detection receiver is affected even by a method for constructing the system matrix for use in the joint detection receiver. However, the complexity of the joint detection receiver is dependent on the method of construction of the system matrix for use in the joint detection receiver.
According to conventional arts associated with the aforementioned joint detection receiver, only a method for constructing the system matrix for joint detection receivers with the same spreading factors in the same time slot in the form of a block-circulant matrix has been proposed. As previously stated in the NB-TDD communication system, the NB-TDD communication system may transmit orthogonal codes having different lengths in the case of uplink transmission. The above conventional art must construct different system matrices consistent with lengths of individual channel codes used for the UL transmission to create a system matrix to be used for UL transmission of the NB-TDD communication system. In this way, provided that different system matrices consistent with lengths of individual channel codes are constructed, an internal structure of the joint detection receiver may be more complicated. In conclusion, a new method for efficiently constructing the system matrix when orthogonal codes having different lengths are used for the same time slot must be developed.